Wireless communication stations employing multiple antennas for transmitting and receiving data are known as multiple-input-multiple-output (MIMO) systems The use of multiple antennas offers significant improvement in data throughput and link range without requiting additional bandwidth or increased transmission power MIMO systems exhibit better spectral efficiency than conventional single antenna systems, while having more reliable links and reduced fading. As implied by the name, typical MIMO communication stations employ a plurality of antennas at the transmitter and receiver. One basis for the improvements offered by MIMO systems is the leveraging of the multi-path environment in which such systems are often used. Accordingly, each signal experiences multipath propagation allowing multiple orthogonal channels to be generated between the transmitter and receiver. In turn, data is transmitted simultaneously in parallel over those channels, without requiring more bandwidth.
Despite the advantages represented by MIMO communication systems, there are areas where performance could be improved. As discussed above, multiple independent propagation paths between stations are necessary to create the multiple orthogonal channels that enable the multistream improvements in data throughput. However, an imbalance in the relative path loss between the multiple propagation paths can lead to conditions where the spread in power of the multiple signals arriving at the receiver exceeds the receiver's dynamic range
Since the receiver typically employs an automatic gain function to adjust the total in-band power to a desired level, a signal experiencing relatively minimal path loss will be correspondingly stronger and leveled appropriately. On the other hand, a signal experiencing relatively greater path loss will be weaker and may be closer to the receiver's noise floor, causing it to have a poor signal-to-noise ratio Indeed, when the path loss imbalance between signals is large enough, the dynamic range of the receiver is overcome and it will be unable to simultaneously recover both the strong and weak signal properly. As a result, the transmitter is forced to use a lower modulation rate or lower number of independent streams of simultaneous data, thus reducing the PHY data throughput.
As will be appreciated, the transmit antenna gains themselves may also be subject to an imbalance that can lead to this condition. Short range, line-of-sight (LOS) channel conditions typically have relatively equal losses for the direct and cross paths of the transmit and receive antennas. However, physical orientation and location of the independent antennas are major factors impacting antenna gain in a MIMO system. As a result, the antennas can exhibit significant imbalances in gain with resulting impaired performance in LOS conditions. Thus, even though there is a relatively balanced channel with respect to path loss, the power into the channel is already imbalanced due to the mismatch in transmit antenna gain and the signals arrive at the receiver with a large spread in power This causes the same type of stress to the receiver's dynamic range as the imbalanced path loss condition described above.
An example of this condition is shown in FIG. 1, which schematically represents the effects of imbalanced transmission antenna gain over a balanced channel from Node A to Node B in a 3×3 MIMO system. The baseband delivers representative pulse waveform signals 100, 102 and 104 indicated by power versus frequency plots having substantially equivalent power levels into three transmit chains 106, 108 and 110. Due to imbalanced antenna gain, indicated by transmit antenna gain block 112, signals 116 and 118 are attenuated relative to signal 114. Each chain is transmitted over a balanced channel in block 120. Correspondingly, the signal sent by each transmit chain arrives at receive antenna gain block 122, shown with balanced antenna gain. Thus, each receive chain 124, 126 and 128 receives a composite of the signals from each transmit chain, indicated with respect to receive chain 124 by signal 130. As can be seen, the components of signal 130 do not have equivalent power levels and the spread potentially exceeds the dynamic range of the receiver or otherwise degrades its performance. Although not shown, the same conditions exist in the received composite signal at the other two chains.
Through testing, it has been determined that the imbalanced antenna gain transmission impacts the far side receiver but imbalanced antenna gain at receive is compensated for by automatic gain control typically present in a receiver as shown in FIG. 2 Signals 200, 202 and 204, having substantially equivalent power levels are fed into three transmit chains 206, 208 and 210 for transmission from Node B to Node A. The signals have a balanced antenna gain during transmission, as indicated by transmit antenna gain block 212. The signals are transmitted over a balanced channel in block 214. As such, the components of composite signals 216, 218 and 220 arriving at Node A also have substantially equivalent power levels. The signals at Node A experience an imbalanced antenna gain, as indicated by receive antenna gain block 222 and are fed into receive chains 224, 226 and 228. Due to the imbalanced gain, composite signals 232 and 234 are attenuated relative to composite signal 230. However, the receiver's independent automatic gain control (AGC) for each chain compensates for this attenuation in AGC block 236, delivering equalized composite signals 238, 240 and 242 to the baseband
One conventional strategy for compensating for imbalances in transmit channels for MIMO communications systems relies on beamforming techniques which seek to adjust characteristics of the signal broadcast from each antenna to focus the transmitted energy at the receiver However, beamforming techniques necessarily require knowledge about the characteristics of the communication channels to allow the appropriate adjustments to be made to the transmission signals and correspondingly require relatively complicated channel estimation strategies, such as determination of an appropriate steering matrix.
Accordingly, it would be desirable to provide systems and methods for wireless communication that minimize the dynamic range experienced by the receiver to avoid reductions in data throughput. Similarly, it would be desirable to provide systems and methods that are able to compensate for imbalanced antenna gains without incurring the complexity associated with beamforming. This invention satisfies these and other needs.